214 



STUDY OF YOUNG CHICK EMBRYOS. 



The lower cavity is the anlage of the allantois, All, which is destined to grow out 

 during the next few days into a relatively large round vesicle. The tissue on the 

 ventral side of the spinal^ cord, Sp.c, is connected by a band of cells with the 

 wall of the caudal intestine, cau.i. If the sections just in front are studied care- 

 fully, it can be easily observed that the notochord also passes over without boun- 

 dary into the same band of cells, which is a mass representing the fusion of the 

 walls of the medullary canal of the intestine and of the tissue of the notochord. 

 In this fused tissue we can, with our present means, detect no signs of the corning 

 differentiation. Just as the walls of the caudal intestine are fused with the tissues 

 on the dorsal side, so also are they fused on the ventral side' with the tissue of 

 the allantois. If we follow the tissues laterally, we see that they merge into the 

 mesoderm proper. From the mesoderm there h^,s been a distinct upgrowth of 



Cos. EC. Mes. Sp.c. nch. S.z. 



Som. 



mes.' Ent. 



All. 



Ve. msth. Spl. 



FIG. 163. SECTION OF A CHICK EMBRYO WITH TWENTY-EIGHT SEGMENTS. TRANSVERSE SERIES 92, SECTION 427. 

 All, Allantois. Cce, Coelom. EC, Ectoderm. Ent, Entoderm. Mes, Mesoderm. mes', Splanchnic mesoderm. 



msth, Mesothelium. nch, }STotochord. Som, Somatopleure. Sp.c, Spinal cord. Spl, Splanchnopleure. 



S.z, Segmental zone of mesoderm. Ve, : Blood-vessel. X 50 diams. 



tissue of rather loose texture on either side of the medullary canal to form the 

 segmental zone, 'S.z. 



Section through the Allantois behind the Intestine (Fig. 163). This section is 

 onjy three in the series beyond that last described, yet it is posterior to the caudal 

 intestine and shows, therefore, more completely the fusion of the structures in 

 the axial region. Except for the absence of the caudal intestine, the description 

 of the last section might apply also to this. The shape of the spinal cord, Sp.c, 

 is somewhat different, and its merging on the ventral side with the underlying 

 tissues is more marked. The cavity of the allantois is smaller and almost slit- 

 like. The other differences do not call for special description. 



Horizontal Section (Fig. 164). The student will find it profitable to make a 

 series of sections in the horizontal plane, trying to cut them as nearly as possible 

 parallel with the median plane of the fore-brain and mid-brain. 



The accompanying figure 164 is from a section of such a series. It shows 

 very clearly the general form of the embryo, the curvature of the neck, the sharp 

 angle of the head-bend, and the almost straight body. In the section represented 

 the lung stretch of the cavity of the fourth ventricle or hind-brain, Ven. IV. \< 



